The invention relates generally to the printing of digital postal indicia, and relates in particular to approaches for the non-contact measurement of velocity of a mail piece using interference patterns created by beams of coherent light.
For many decades it has been routine to print postal indicia by means of relief printing dies. By xe2x80x9crelief diesxe2x80x9d is meant dies in which the high points receive ink which is transferred to a mail piece. This is contrasted to intaglio print elements in which ink is applied to the entirety of the printing plate and removed from the high points, leaving ink only in the low points to be transferred to the paper. The relief printing die offers many advantages, among them that the image quality is very good due to the pressure applied by the die upon the mail piece, which tends to keep the mail piece captive and reduce the possibility of unwanted and unintended motion of the mail piece relative to the printing die. A person who might attempt to print postal indicia without paying for them would be faced with the task of creating a counterfeit printing die, or with the task of tampering with a postage meter (franking machine) to force its printing die to be used to print postage indicia that are otherwise unaccounted-for. The latter approach is unsatisfactory because the design of the postage meter is such that tampering is easy to detect through visual examination of the meter.
In such a postage meter there are accounting registers which account for postage indicia that are to be printed or that have been printed. For example, in some countries there will be a xe2x80x9cdescending registerxe2x80x9d and an xe2x80x9cascending registerxe2x80x9d. The former keeps track of the postage value that was paid for in advance, and when the descending register drops to some predetermined level the meter refuses to print any more postage. The latter keeps track of the total amount of postage that has ever been printed on the postage meter. The accounting registers and the printing mechanism are all within a single secure housing, and this provides a confidence level that if a postage indicium has been printed, it has been accounted for in the accounting registers. The communications between the accounting registers and the printing mechanism are secure communications because of the secure housing.
The die printing is done with fluorescent ink which provides yet another confidence level against counterfeit postal indicia.
In recent years it has been proposed by some postal authorities to print postal indicia by means of digital printing methods such as ink jet and thermal transfer, and by the use of commonly available inks and transferred pigments. With such a digital printing method the print area is typically bit-mapped, and the mail piece typically moves relative. to the print head. As the mail piece moves relative to the print head, a bit-mapped data stream is communicated to the print head and ink or transferred pigment are deposited on the mail piece in response to the bit-mapped data stream. With many such proposed systems there is no physically secure communications channel between the accounting registers and the printer.
Of course it will be appreciated that if a commonly available printer (and ink or pigment) is used, there is a substantial risk that some persons will be tempted to avoid having to pay for postage by the step of printing counterfeit postal indicia on mail pieces. This is particularly easy to do since the printed indicium could be scanned in a commonly available image scanner to arrive at a bit-mapped image that would, when printed, look quite like the original. Furthermore it will be recalled that in many such systems the communications channel between the accounting registers and the printer is, by definition, insecure. Thus the would-be counterfeiter can simply intercept the bit image of the postal indicium on its way from the accounting registers to the printer. This interception may be done in software (for example through the operating system) or in hardware (for example by capturing electrical signals passing through the Centronics-standard parallel printer cable).
The one measure that has been proposed to provide some level of protection against counterfeit postal indicia when commonly available printers are employed is the use of cryptographic authentication. The assumption is that there is a secure housing somewhere in the system, and within this housing are the accounting registers and also a cryptographic engine. The cryptographic engine is used, for example, to cryptographically xe2x80x9csignxe2x80x9d the postal indicium. The post office may then examine the cryptographic signature on the mail piece and determine whether the indicium is authentic or counterfeit.
While the approach of the use of cryptographic signatures and commonly available printers is attractive from a theoretical point of view, there are practical drawbacks. It is easy enough to say that the indicium will include information that is to be examined by the post office, but on a practical level this will work only if the indicium, including the cryptographic signature, is machine-readable. It would be possible to use OCR (optical character recognition) characters that are optimized for scanning and recognition, or to use a bar code, for example a two-dimensional bar code. The US Postal Service has proposed the use of a two-dimensional bar code. The assumption is that nearly all mail pieces would be scanned and their indicia authenticated. This would require consistency checking for each indicium (e.g. that the cryptographic signature is consistent with the information that is xe2x80x9csignedxe2x80x9d, such as the date and meter ID number). This would also require duplicate checking to ensure that a particular indicium has not been used more than once, since presumably the system is set up so that each indicium is supposed to be unique. The information proposed to be communicated by means of the two-dimensional bar code amounts to many hundreds of bits of data. The postal indicium thus would comprise a very large bar code as well as human-readable information that approximates a postal indicium of the type that is historically familiar.
Those with experience with postage meters will readily appreciate that a postal indicium which contains the images of a historically familiar indicium and that also contains a two-dimensional bar code of several hundred bits is quite sizeable and, importantly, is at risk of being smudged or otherwise damaged. If an inkjet printer is used, there is the concern that the indicium would be touched or smudged before the ink has dried. There is the further concern that if the indicium gets wet (for example, if the envelope is exposed to rain or other moisture) then the ink may smudge. In the case of a thermal transfer image, there is the concern that the thermally transferred pigment may be removed by abrasion or other perils. There is also the concern that the mail piece may not be perfectly constant in thickness, for example, if the envelope contents do not completely fill the envelope or if there is a staple or paper clip in the area where the indicium will be printed. These factors all work against the possibility that the two-dimensional bar code can be successfully read by the post office for reason of authentication.
Even if none of these perils occursxe2x80x94no moisture, no smudging, no abrasion, no paper clip or staplexe2x80x94there is still the problem that the two-dimensional bar code must be printed faithfully in the first place. The horizontal and vertical spacing of the pixels that make up the bar code is required to be maintained accurately. This requirement applies to each pixel individually and there is the related requirement that the pixels be consistent in size across the vertical and horizontal extent of the bar code.
As will be appreciated, it would be very convenient if the designer of the digital printing franking machine were able to assume that the mail piece were always moving at an exact and very predictable velocity relative to the print head. In such a case, the data stream communicated to the print head could be clocked at a particular fixed rate, yielding an image in which everything is controlled and the image has all desired qualities.
As a general matter, however, the designer of the digital postage flanking machine is not able to assume that the mail piece is always moving at an exact and very predictable velocity relative to the print head. There can be variations of speed for mail pieces in the paper path depending on the number and types of mail pieces and their sizes. Many factors can contribute to the variations, such as the thickness of particular mail pieces and changes in the total mass of mail pieces that are within the paper path at a particular moment. A variation of speed over a relatively long interval relative to a desired speed can give rise to a postal indicium that is squeezed or stretched in its entirety in the axis parallel to the direction of motion of the mail piece. In such a case a printed pattern that is intended to form a circle would instead form an ellipse. On the other hand, variations of speed over relatively short intervals can give rise to a postal indicium that is irregular in its pixel dimension along the direction of motion. Any of these distortions of the bar code risks making the bar code unreadable for authentication purposes.
A commonly used approach for measuring the velocity of a mail piece is to place a roller in friction contact with the mail piece. The roller is coupled with a resolver or other sensor, and the resolver output is used to clock the print bit-map into the print head. This approach is not completely satisfactory, however. The roller may slip relative to the mail piece. The roller and the other moving, parts coupled to it present a rotational inertia which make it difficult for the roller to keep up with sudden changes in the velocity of the mail piece. The roller is also a maintenance item and the pressure with which it is biased toward the mail piece may need to be adjusted from time to time.
It is desirable to have a reliable way of measuring the velocity of a mail piece that is sufficiently accurate, lacks the drawbacks of the roller approach, and is not too expensive.
A postage meter (franking machine) uses a digital print head such as an ink-jet or thermal transfer or dot-matrix print head, for which it is necessary to know the velocity of the mail piece passing by the print head. Two collimated monochromatic beams strike the mail piece, one at an angle leading the mail piece velocity and the other at an angle lagging the mail piece velocity. The beams converge yielding a sensing region filled with a diffraction pattern. The mail piece, assumed to be rough at a scale that is appropriate for the velocity measurement, moves at some velocity. A detector detects light intensity (photon flux) at a small region within the sensing region, and the intensity signal has a frequency that is proportional to the mail piece velocity. The frequency is detected or measured, the instantaneous velocity is derived therefrom, and the velocity is used to control the print head. In this way a two-dimensional print image (postage indicium) is faithfully printed on the mail piece with minimal distortion even in the event of non-constant velocity of the mail piece.